Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	4,976,882	5,227,512
Total	4,976,882	5,227,512
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	147,809	149,669	$ 181,764
Other Assets	58,930	52,342
Total assets	28,277,282	29,169,468
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	3,983,877	4,235,312
Total	3,983,877	4,235,312
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,279,994	5,539,964
Restricted Cash	147,649	149,412
Other Assets	300,468	312,763
Total assets	5,728,111	6,002,139
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	3,983,877	4,235,312
Total	3,983,877	4,235,312
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,279,994	5,539,964
Restricted Cash	147,649	149,412
Other Assets	300,115	311,697
Total assets	5,727,758	6,001,073
Variable Interest Entity, Primary Beneficiary | Secured Borrowing Facility
Debt Outstanding
Short-Term	0	0
Long-Term	0	0
Total	0	0
Carrying Amount of Assets Securing Debt Outstanding
Loans	0	0
Restricted Cash	0	0
Other Assets	353	1,066
Total assets	$ 353	$ 1,066